1. Field of Invention
The present invention relates to a light-emitting device, and more particularly, to an organic electroluminescent (OEL) device and pixel structure, array and driving method thereof.
2. Description of Related Art
Along with the advancement of multimedia information communication technology, the importance of displays in providing human the access to computer interface has increasingly acknowledged. The flat panel display (FPD) has become the mainstream product in the market because of high-quality resolution, efficient space utilization, low power consumption, and the freedom of radiation exposure.
The so-called flat panel displays include liquid crystal displays (LCD), organic electroluminescent displays, and plasma display panels (PDP), etc. The organic electroluminescent displays are self-emitting and can be divided into two groups, small molecular OLEDs (SMOLEDs) and Polymer OLEDs (Poly-OLEDs), depending on the molecular weights of the organic electroluminescent materials. The organic electroluminescent displays have the advantages of wide viewing angle, low production cost, faster response and higher switching speed (above 100 times faster than that of liquid crystal displays), low power consumption, and widespread applications in current-driven portable machines, wide temperature latitudes, light weight, and design versatility such as: miniaturization and slimness, to meet the requirements for displays in the Multimedia Age. Therefore, the organic electroluminescent displays have a great potential for becoming the next-generation flat-panel display medium.
The organic electroluminescent displays may be classified into active organic electroluminescent displays and passive organic electroluminescent displays, depending on the driving method. Currently, the active organic electroluminescent displays are most widely used thin film transistors (TFT) as the actuators for driving the display devices. FIG. 1 is a circuit diagram illustrating part of a conventional active organic electroluminescent display. Referring to FIG. 1, in the conventional active organic electroluminescent display 100, in general, two thin film transistors T1, T2 and a capacitor C are disposed in each pixel to drive a pixel electrode 106 to allow the organic electroluminescent device (not shown herein) to emit light. The pixel electrode 106 (the anode of the organic electroluminescent device) is transparent electrode; therefore, the light emitted from the organic electroluminescent device can be transmitted through the pixel electrode 106 to outside of the display. The actual emission area of the pixel shown in FIG. 1 merely equals the area of the pixel electrode 106, and light can not transmit through the area outside of the pixel electrode 106 (the area disposed with thin film transistors T1, T2 and capacitor C). Therefore, the aperture ratio of the organic electroluminescent display 100 is limited by the sizes of the thin film transistors T1, T2 and capacitor C. For wider aperture ratio of the active organic electroluminescent display 100, an increase in the area of the pixel electrode 106 is necessary but the process reliability is lowered accordingly.
To solve the problem discussed above, a top emission active organic electroluminescent display is provided conventionally. FIG. 2 is a profile view illustrating part of a conventional top emission active organic electroluminescent display. Referring to FIG. 2, when the anode 206 (i.e. the pixel electrode) of the active organic electroluminescent display 200 is a common metal electrode, and the cathode 210 thereof is a transparent electrode, the light emitted from the organic functional layer 208 may be transmitted through the cathode 210 along the direction 212 shown in FIG. 2. Since the active organic electroluminescent display 200 emits light from the top, the aperture ratio of the organic electroluminescent display 200 is not subject to the size of the actuator 204.
However, there are uncertain factors in the process with possibly a decrease in process reliability when the organic electroluminescent devices are driven by thin film transistors.